The present invention relates to a lithium ion secondary battery and a charging method therefor. In particular, the invention relates to a lithium ion secondary battery which is charged with a simple operation and at low cost by making use of a charging device having a simple circuit configuration, and a charging method therefor.
A lithium ion secondary battery has characteristics of having large capacity and high energy density, exhibiting an excellent charge-and-discharge cycle characteristic, and being capable of maintaining a rated output for a long period of time. Therefore, lithium ion secondary batteries are widely used as driving batteries for a variety of devices, such as a cellular phone, a notebook computer, or a PDA.
Constant-current and constant-voltage charging is usually employed for charging a lithium ion secondary battery. In the constant-current and constant-voltage charging, first, a battery is charged with constant current until a voltage reaches a predetermined upper limit voltage; and thereafter, the voltage is maintained at the predetermined upper limit voltage. When the battery is set a constant-voltage mode, a current value decreases. Hence, charging is completed at a point in time where the current value falls to a given current value (see, e.g., JP-A-5-111184).
However, since a lithium ion secondary battery employs a nonaqueous electrolyte solution having high electrical resistance as a solvent of an electrolyte solution, a problem that full charging of a lithium ion secondary battery requires a long period of time has arisen.
Conceivable methods for reducing time required for charging a lithium ion secondary battery through constant-current and constant-voltage charging include a method of setting a charging current value to a high value during constant-current charging, and a method of setting an upper limit voltage value to a high value during the same.
However, when a lithium ion secondary battery which employs LiCoO2 as a positive electrode active material is charged with excessively high current or excessively high voltage, there arises a problem that desorption of Li from the positive electrode active material occurs to an excessive extent, thereby destroying lattices, and deteriorating the charge/discharge cycle characteristics.
Accordingly, when a lithium ion secondary battery employs a carbonaceous material as a negative electrode active material, the battery must be charged as follows. That is, an upper limit voltage value is set to +4.2 V; the battery is charged with constant current of 1 CA (a current value corresponding to a rated capacity value (Ah) of the lithium ion secondary battery) or less; and thereafter, the battery is charged with constant voltage while the voltage value is controlled at a high accuracy of +4.2±0.05 V In addition, when the negative electrode active material is lithium titanate, the upper limit voltage value must be set to +2.7 V, and the voltage value during constant-voltage charging must be controlled at a high accuracy of +2.7±0.05 V
As described above, in the related-art method for charging a lithium ion secondary battery, monitoring of not only a voltage value but also of a current value is indispensable, thereby raising a problem of charging operation being complicated.